In the production of nuclear grade uranium hexafluoride, crude UF.sub.6 containing small amounts of vanadium, titanium and to a greater extent molybdenum are vaporized and fed into a boiler still for processing. While the low boiling impurities (i.e., HF, VOF.sub.3) are easily removed before the distillation of UF.sub.6 commences, the molybdenum contamination remains. Continuous batch-to-batch refinement of crude uranium hexafluoride results in the accumulation of molybdenum impurities in the high boiler still bottom until co-distillation of the molybdenum impurities with the UF.sub.6 takes place. The uranium hexafluoride product made when co-distillation occurs fails to meet regulatory specifications and therefore must be stored as process waste or disposed of in a low level radioactive waste burial site. The elimination of these two alternatives via recycling would enhance the economics of production.
Conventional separation methods for recovering uranium values from acid leach liquors are known. Generally, these processes utilize either ion exchange or solvent extraction technology. Problems are encountered in regard to product purity particularly in the presence of high molybdenum and fluoride ion concentrations which are present in a process-by-product stream and therefore these methods are incompatible in such processes.
The prior art has recognized a need for removing molybdenum from uranium. However, prior art methods have not addressed the problem of molybdenum recovery. Such methods include the ion-exchange work of Fox et al. in U.S. Pat. No. 3,790,658; Ruiz et al. U.S. Pat. No. 4,092,399 or Kuehl et al. in U.S. Pat. No. 4,304,757. The limitations of these methods for reclamation of metal values in a different process waste are pronounced. For example, it is well known that molybdenum will react similarly to uranium in forming anionic complexes which will be adsorbed on resins. As the molybdenum values continue to increase on the resin, a decrease in the total uranium capacity results. In practice efforts are made to control the molybdenum concentration by blending ore leachates to keep the levels of molybdenum in the range of 0.01 to 0.02 grams per liter. In dealing with process waste of a different system, concentrations of up to 50% by weight of molybdenum is present. Clearly, dilution to achieve separation by these methods is not economical. Of much greater concern is the high fluoride ion concentration produced during hydrolysis of the process waste. It has been bound that premature breakthrough of uranium and molybdenum occurs when high concentrations of fluoride ion (&gt;=1000 ppm) is present. This inevitably results in an unacceptably high cross contamination factor for the reprocessed metals as well as a final product which contains fluoride ion exceeding the tolerance limits. An alternative process must obviously be designed.
Attempts to use conventional solvent extraction technology such as that described in U.S. Pat. No. 4,011,296 results in limited practicality. High concentrations of molybdenum in the process waste build up in the amine extractant and act as a "poison" in a manner similar to that observed with the ion exchange resins. This usually occurs when the concentration of molybdenum exceeds 0.03 g per liter of organic phase. Eventually, a maximum tolerance level is reached after which point precipitation of complex amine heteropolymolybdates occurs. The precipitate forms at the organic-aqueous interface as a gummy mass which seriously interferes with the operation. Compounding the problem is the high fluoride ion concentration present in our waste. The formation of uranyl fluorides is possible thus causing these complexes to be retained in the organic phase during stripping. The result is an unacceptable cross-contamination level in the recovered metals. The high fluoride ion concentration also augments the problem by interfering with a rapid phase separation.
An attempt to use the precipitation technique disclosed by Crossley in U.S. Pat. No. 4,393,028 was found to be incompatible for use with a different process waste. Undesirable cations deleterious to the recycling of the uranium values are introduced, the conversion of molybdenum values molybdenum trioxide are not possible, and all products become cross contaminated with fluoride containing compounds to an extent which prevents recycling or resale of the product.
It is therefore apparent, that a means to recover the molybdenum and uranium values from process hydrolysate containing high molybdenum and fluoride ion concentrations would be most desirable. Accordingly, it is an object of this invention to provide a process for the separation of the metal values from the high boiler still bottom residue insufficient purity to allow the uranium values to be recycled in the UF.sub.6 process and the molybdenum values to be reclaimed as molybdenum oxide for subsequent sale to the metals trade (i.e., manufacture of stainless steel, etc.).